In shell investment molding a pouring cup may be created at the entrance of the sprue, as illustrated in the Metals Handbook. The solid wax preform shown in the Handbook is undesirable inasmuch as it involves a separate forming step for the preform and causes a lot of wax to be recycled through the process. Also, as shown, the pouring cup or pouring basin formed by this method usually has a thick edge which is fragile and friable and often causes refractory material to fall into the mold.
For this and other reasons, preformed ceramic pouring cups are now being used. These are attached to the shell mold after the shell mold has been formed and are destroyed when the shell mold is removed from about the molded article. This is an expensive practice but is considered justified because of the other advantages offered by the preform. The preform has a stout outer lip which tends to be resistant to damage and crumbling into the mold and the preform and the lip can be used to position and move the shell mold about during the subsequent handling.
The lips of the preformed pouring cups are flat, i.e. the surface of the lip is in a plane perpendicular to the axis of revolution of the pouring cup. This is probably because of the way they are made in that the ceramic is fired while the preform is supported on its lip on a flat surface. Because the lips of the preforms are flat, they tend to allow debris that may come to rest on the lip to find their way into the mold cavity.